With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries or cells having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Under these circumstances, lithium ion secondary batteries having advantages such as a high charge/discharge voltage and a large charge/discharge capacity have been noticed.
In the lithium ion secondary batteries, in recent years, it is known that lithium titanate is used as a negative electrode active substance (Patent Document 1).
It is known that the lithium titanate: Li4Ti5O12 provides a negative electrode active substance having a high structural stability and a high reliability because the lithium titanate exhibits a much less change in crystal structure even when subjected to insertion and desorption reactions of lithium ions upon charging and discharging operations.
Hitherto, there is known a so-called solid state reaction method (dry method) as the method for producing lithium titanate (Li4Ti5O12) in which mixed particles prepared by dry-mixing or wet-mixing a lithium salt and an oxide of titanium such that an Li/Ti ratio therein is 0.80 (a simple mixture of the lithium salt and the oxide of titanium) are heated and calcined to obtain Li4Ti5O12 (Patent Documents 1, 6, 8 and 9).
On the other hand, there is also known the method (wet-method) including a liquid phase reaction and a solid state reaction in which a mixture of titanium and lithium is subjected hydrothermal treatment and then heated and calcined to obtain Li4Ti5O12 (Patent Documents 3 and 4).
In addition, in Patent Document 5, it is described that as a result of XRD of lithium titanate, a peak intensity ratio of TiO2 to Li4Ti5O12 and a peak intensity ratio of Li2TiO3 to Li4Ti5O12 both are not more than 7, preferably not more than 3 and more preferably not more than 1, and that as the amounts of these impurity phases are reduced, a diffusion rate of lithium ions is increased and an ionic conductivity and a heavy-current characteristic (high-efficiency discharge capacity retention rate) thereof are enhanced.
Also, in Patent Document 5, it is described that as the crystallite size of the lithium titanate is reduced or as the amounts of these impurity phases are reduced, a diffusion rate of lithium ions is increased and an ionic conductivity and a heavy-current characteristic (high-efficiency discharge capacity retention rate) thereof are enhanced.
Further, as a method similar to the above solid state reaction method, there is also known the production process in which a slurry comprising an oxide of titanium, a titanic acid compound (such as m-titanic acid, o-titanic acid or a mixture thereof) and a lithium salt is dried and granulated, and then heated and calcined (Patent Documents 10 and 11).
On the other hand, there is also known the process for producing lithium titanate (Li4Ti5O12) comprising a step of reacting a titanium compound with an ammonium compound in water to obtain a titanic acid compound; a step of reacting the titanic acid compound with a lithium compound in water to obtain lithium titanate hydrate; and a step of dehydrating the lithium titanate hydrate under heating (Patent Document 3).
Also, the lithium titanate (Li4Ti5O12) has such a problem that a high-efficiency discharge capacity retention rate thereof is low owing to a high electrical insulating property thereof.
On the other hand, it is known that a part of lithium and/or titanium of lithium titanate (Li4Ti5O12) is substituted with a transition metal such as Fe (Patent Document 12) and Cu (Patent Document 13) or the other metals (Patent Documents 14 to 16) in order to improve various properties of the lithium titanate.
In addition, in Patent Document 17, there is described the invention concerning an active substance for a lithium ion secondary battery which has a composition represented by the formula: Li[Li(1-2x)/3MgxTi(5-x)/3]O4 (0<x≦½).